The present invention relates generally to extrusion dies, and more particularly to apparatus for controlling material flow through the die.
Extrusion processes have long been used to produce sheets or webs of formable material, such as thermoplastic. These processes generally involve forcing a viscous material through a die typically comprising an inlet, a cavity, and an exit. As the material passes through the die, it is often necessary to influence the flow to obtain a product at the die exit having desirable characteristics.
Past methods of influencing material flow have included the use of a flow control device mounted in a channel within a die cavity, such as a restrictor bar, a flexible membrane, or another insert. Since most extrusion processes operate under high pressure, leakage of material into spaces between the insert and the channel is a significant concern. Build-up of material in the channel can prevent proper flow control and result in burning of material therein. Hence, the channel and insert must be designed and machined to tolerances that will ensure a tight sealing fit and thereby prevent leakage.
The assignee of the instant application currently offers for sale a die having a flexible membrane mounted by spaced mounting members in correspondingly-shaped tapered channels in the die cavity. Bolts extend into the mounting members and are tightened to cause the members to seal against walls forming the tapered channel. End surfaces of the membrane are clamped between body members of the die to form a seal between the die and the membrane.
While this arrangement of elements is effective to accomplish flow control, some drawbacks are encountered. Specifically, the installation process is complicated and expensive. Further, machining of the tapered channels is difficult to accomplish with the required degree of precision and is not well suited (due to the channel configuration) to large-scale production techniques. In addition, re-working of such a die would likely result in a substantial amount of refitting of the flow control device in the channels. Manufacture of an aftermarket flow control device without the original tooling would also be problematic. Furthermore, clamping the end surfaces of the membrane can restrict adjustment of the membrane which, in turn, can make it more difficult to achieve a desired flow control.
According to one aspect of the present invention, a flow control device includes a flexible membrane movable in a certain direction and having a built-up end portion, a gasket disposed adjacent the end portion, and means for causing the gasket to apply a force against the built-up end portion in a direction transverse to the certain direction such that the gasket seals against the flexible membrane.
Other advantages of the invention will be apparent to those skilled in the art from the following detailed description taken in conjunction with the drawings and the appended claims.